An "ofproto provider" is what ofproto uses to directly monitor and
control an OpenFlow-capable switch. struct ofproto_class, in
-ofproto/private.h, defines the interfaces to implement an ofproto
-provider for new hardware or software. That structure contains many
-function pointers, each of which has a comment that is meant to
+ofproto/ofproto-provider.h, defines the interfaces to implement an
+ofproto provider for new hardware or software. That structure contains
+many function pointers, each of which has a comment that is meant to
describe its behavior in detail. If the requirements are unclear,
please report this as a bug.
latter is equivalent to the one of the former, but the difference in
name makes the intended use obvious.
-ovs-vswitchd is the most sophisticated of ofproto's clients, but
-ofproto can have other clients as well. test-openflowd, in the
-"tests" directory, is much simpler than ovs-vswitchd. It may be
-easier to initially bring up test-openflowd as part of a port.
-
lib/entropy.c assumes that it can obtain high-quality random number
seeds at startup by reading from /dev/urandom. You will need to
modify it if this is not true on your platform.
Linux. Optionally you may implement them for your platform as well.
+Why OVS Does Not Support Hybrid Providers
+-----------------------------------------
+
+The "Porting Strategies" section above describes the "ofproto
+provider" and "dpif provider" porting strategies. Only an ofproto
+provider can take advantage of hardware TCAM support, and only a dpif
+provider can take advantage of the OVS built-in implementations of
+various features. It is therefore tempting to suggest a hybrid
+approach that shares the advantages of both strategies.
+
+However, Open vSwitch does not support a hybrid approach. Doing so
+may be possible, with a significant amount of extra development work,
+but it does not yet seem worthwhile, for the reasons explained below.
+
+First, user surprise is likely when a switch supports a feature only
+with a high performance penalty. For example, one user questioned why
+adding a particular OpenFlow action to a flow caused a 1,058x slowdown
+on a hardware OpenFlow implementation [1]. The action required the
+flow to be implemented in software.
+
+Given that implementing a flow in software on the slow management CPU
+of a hardware switch causes a major slowdown, software-implemented
+flows would only make sense for very low-volume traffic. But many of
+the features built into the OVS software switch implementation would
+need to apply to every flow to be useful. There is no value, for
+example, in applying bonding or 802.1Q VLAN support only to low-volume
+traffic.
+
+Besides supporting features of OpenFlow actions, a hybrid approach
+could also support forms of matching not supported by particular
+switching hardware, by sending all packets that might match a rule to
+software. But again this can cause an unacceptable slowdown by
+forcing bulk traffic through software in the hardware switch's slow
+management CPU. Consider, for example, a hardware switch that can
+match on the IPv6 Ethernet type but not on fields in IPv6 headers. An
+OpenFlow table that matched on the IPv6 Ethernet type would perform
+well, but adding a rule that matched only UDPv6 would force every IPv6
+packet to software, slowing down not just UDPv6 but all IPv6
+processing.
+
+[1] Aaron Rosen, "Modify packet fields extremely slow",
+ openflow-discuss mailing list, June 26, 2011, archived at
+ https://mailman.stanford.edu/pipermail/openflow-discuss/2011-June/002386.html.
+
+
Questions
---------
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